Recently, it is demanded in a market to further extend a service life of a tire by improving a durability thereof. Natural rubber is generally superior to a synthetic rubber in a tensile strength, and is used in various skeletal members of the tire such as a tread, a sidewall, a carcass ply, a bead filler and the like. As a countermeasure on the marketing demand for improving the durability of the tire to extend its life, therefore, there is an improvement in the tensile strength of the natural rubber used in the various skeletal members of the tire.
The molecular weight of the natural rubber is inherently very high such as RSS. However, when the viscosity is too high in the preparation of a rubber composition by compounding with a filler in a mixer, the processability is deteriorated and the dispersion of the filler is also deteriorated, so that the natural rubber is actually used after being previously peptized by mastication or the like. As a result, the molecular weight of the natural rubber is lowered by the peptization and the tensile strength inherent to the natural rubber is not developed sufficiently (see JP-A-2005-162810).
Moreover, among the natural rubbers, TSR is low in the viscosity and relatively good in the processability as compared with RSS. However, since TSR is dried at a high temperature in a short time, the molecular chain is broken and the tensile strength is not high enough.